Nearly two dozen people could be facing fines of £10,000 each for organising an illegal rave on the edge of the Brecon Beacons.
South Wales Police have been trying to disperse the rave since Sunday afternoon when around 3,000 people congregated at a former opencast coalmine near the village of Banwen.
By Monday morning, around 400 people remained, the force said, but it added it was having difficulty flushing them off the 4,000 acre site despite confiscating several sound systems.
Police said 22 people have been reported for summons, alleged to have organised the rave..
Under newly introduced legislation from the Welsh Government to enforce social distancing, those convicted or organising an unlicensed music event of more than 30 people can be fined up to £10,000.
A number of vehicles were also seized, while two people were arrested for driving while unfit through alcohol or drugs and a third for a public order offence.
South Wales Police were assisted by officers from Dyfed Powys Police and the British Transport Police, as well as a police helicopter.
By 10.30pm only a handful of ravers remained, but South Wales Police said officers would remain on site until everyone had departed.
Assistant Chief Constable Dave Thorne said: “We responded as soon as we became aware of the event and we have had a police presence on site day and night and I would like to reassure local communities we acted as robustly as we can and we will continue to do so.
“It is extremely disappointing and disrespectful to our communities that we have had to deal with an event like this, not only on a busy weekend for the emergency services, but also during the coronavirus pandemic where legislation has been put in place to keep everyone safe.
“Those gathering know that their actions are irresponsible and many travelled hundreds of miles to attend the event.”
Multiple raves have been shutdown across the country this bank holiday weekend.
Police in Norfolk were pelted with missiles when they broke up a 500-strong rave in Thetford Forest, while in London the Met Police said it had shut down 21 unlicensed music events.
West Yorkshire Police said eight people were fined £10,000 after several parties across Headingley and Burley on Saturday and into the early hours of Sunday.
Meanwhile, thousands of pounds worth of equipment was also seized in the police crackdown in the Chapeltown area of Leeds, and ahead of a planned unlicensed music event in Harlow, Essex on Saturday.